Mouth. I really love the sensation of my partner sucking my balls empty until the last drop and then even further maybe.
She loves it too, and is a master of deepthroating.
When she notices I'm close, she stops a bit and just keeps on swallowing with less sensation than before, but balls deep in her only a snake could compete. This prolongs it a bit, and she can enjoy some precum as long as we both can.
The taste of it (sweet and fruity according to her) drives her crazy, and often she just edges me for another couple minutes, until I just can't hold it no more and let it happen, either until she can't keep up anymore with swallowing, or until it becomes painful.
Vaginal on the other hand, good too, but we use condoms and as soon as I cum in her I have to pull out, because my dick becomes flaccid and the risks become too high.
Don't get me wrong, the vaginal cramping sensation when she cums is just as good, but not as controlled and manageable.
A whirlpool or swimming pool. Doesn't matter if public or private for me. Sadly, both wouldn't work because of hygiene reasons, risk of infections, and of course societal norms and laws.
I think it would be amazing to masturbate on the pool jets first to warm up. Maybe some edging on my side, and a few orgasms by the water jets on her side.
And then continue to fuck or rest in the bubble bath, because I love the tickling sensation the air bubbles give me.
We already had sex a few times in the bathtub at home. I love the bath itself (warm, cozy, and the skin contact feels better), and she loves how the water propagates the pressure from either my hand fingering her or my body pounding to her clit. It's both vaginal and clitoral/ ball stimulation.
Especially in a pool deep enough to stand or swim, I think some interesting positions could be possible to the buoyancy.